What’s the sense of taste for, exactly? It will obviously be useful in telling us that two foods or beverages are different – one more sour or sweet than the other, for example. And we will use taste information to decide whether a food is too salty, not salty enough, or just right. But what were the evolutionary pressures that decided we should have a sensory system that responded specifically to a discrete set of qualities – sweet, sour, salty, bitter, umami (savoury), and perhaps fat. Such a “limited” list of taste qualities can be contrasted with our sense of smell, in which thousands, if not tens of thousands, of individual, distinct qualities can be distinguished.
Clearly, food selection must be an important taste function and clues to the adaptive significance of taste lie in the fact that our hedonic reactions - our likes and dislikes - to basic tastes are remarkably uniform. Hence, even though diets vary dramatically across different cultures, it has been shown that degrees of liking for some basic tastes (especially sweetness, bitterness and umami) outside the context of foods or drinks are highly similar .
This could suggest a common denominator in human diets or alternatively a genetic predisposition to respond hedonically in particular ways to tastes. Some decades ago, Israeli scientist Jacob Steiner undertook landmark studies in which he placed solutions of taste solutions onto the tongues of human newborns. From a series of photographs taken of the babies’ facial expression, it was evident that the sweet taste of sucrose evoked pleasure. The babies can be seen licking their lips and unambiguously smiling. In contrast, a bitter taste provokes a similarly unambiguous gaping mouth and a wrinkled nose, both indicating rejection of the taste.
Steiner’s studies do not provide us with evidence of the precise origin of these fixed facial expressions to tastes. Steiner’s babies were very new – no more than a few hours old – and they were tested prior to being fed for the first time. So, there was no opportunity for them to learn how to respond (indeed, equivalent facial expressions were evident in babies born congenitally blind). It is still possible, of course, that preference for sweet and rejection of bitter were learned as a result of foetal exposure to sweet and bitter substances in the maternal diet. The impact of such exposures on post-natal preferences in now well-established . Whether pre-natal learning or genetic programming is responsible for such innate responses to tastes in humans is not known but, in any case, these responses are important indicators of the functions of taste.
But it is generally considered that the fact that facial expressions to sweet and bitter essentially identical to those of newborn humans are also observed in other species, including rats and apes , tells us that liking sweet and disliking bitter have evolved for a reason. Responding positively to sweet tastes means that we are motivated to consume plants that are high in energy-proving sugars, including fruits. In contrast, bitter substances in nature are often, if not universally, toxic. Hence, disliking bitterness can be protective. As a result, it has been argued that our sense of taste acts as a “guardian of the gut”, screening potential foods for the presence of carbohydrates and the absence of bitter poisons prior to ingestion. Other tastes, too, have been included in this adaptive explanation. Thus, the pleasantness of umami tastes is thought to reflect the importance of umami-producing amino acids in proteins, and our body’s need for sodium is known to underlie our preference for saltiness. Highly acidic (sour) tastes are generally disliked, indicating perhaps a need to avoid unripe or spoiled fruits, both of which are more sour than sweet.
Beyond the presence of consistent across-species facial expressions to taste qualities, there has been little other science that has directly supported this view of taste as an evolved active participant in the food selection process. Another way of looking at this question, though, is to look at the eating habits of different species. If our innate hedonic responses to tastes are crucial to food selection, then they should reflect the variations in diet that exist across different animal species. Just such evidence was suggested by the relatively recent observation that, amongst mammals, it is the cat family that is most obviously indifferent to sweet tastes – just what we might expect from species that do not consume plants of any kind. The vampire bat is also, and unsurprisingly, indifferent to sweet treats.
Now, however, there is new genetic evidence for the evolution of taste as a crucial determinant of food selection. This has come from a recent study by Jiang and colleagues  that has further enhanced our understanding of the direct relationship of taste variations to diets across different species. Searching for the part of the genome responsible for sweet taste receptors (a gene known as Tas1r2) in various species that are either fully carnivorous, as well as others that consume both meat and plants, has revealed the existence of pseudogenization of this gene. In other words, the gene - while present - has lost its ability to generate functioning sweetness receptors. This suggests a process whereby the gene, once active, has become inactive during the evolution of a completely carnivorous species.
In this study, these non-functional sweet taste genes were found not just in carnivorous cats but also in other carnivorous species such as sea lions, seals, otters, hyena and dolphin. The Spectacled bear, on the other hand, eats both meat and plants and retained functioning Tas2r1 genes. Testing for the presence of sweet preferences in some of these species was consistent with the genetic data: otters showed no preferences for sweet tastes, while the Spectacled bear did. One final piece of evidence regarding the link between taste sensitivity and diet was evident in the genome of the aquatic mammals tested. In addition to lacking functional sweet taste genes, neither sea lions nor dolphins showed evidence of functioning genes for either umami or bitter tastes. This is consistent with both species’ effective lack of taste buds, and also behaviourally with their approach to eating, which is to swallow prey whole without any consideration for their taste qualities.
What does this research tell us about us? Firstly, these new data indicate that the fact that humans have functioning taste receptors for sweet, umami and bitter (as well as somewhat different mechanisms for detecting salty and sour) is strong pointer to what we have evolved to consume, namely sugar-containing plants, as well as meat. It tells us, too, that bitterness is (or perhaps has been) reflective of a genuine dietary risk that we have faced during relatively recent evolutionary development. Indeed, other research on this issue has shown that evolutionary development from old-world monkey to new-world monkeys to humans is accompanied by an increasing numbers of functioning bitterness receptors. Taste then has clearly been an important sensory system in human evolution.
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3. Steiner, J.E., et al., Comparative expression of hedonic impact: affective reactions to taste by human infants and other primates. Neuroscience & Biobehavioral Reviews, 2001. 25(1): p. 53-74.
4. Jiang, P., et al., Major taste loss in carnivorous mammals. Proceedings of the National Academy of Sciences USA, 2012. [www.pnas.org/cgi/doi/10.1073/pnas.1118360109]